1. Field of Invention
This invention relates to an NMR imaging apparatus, and more particularly to such an apparatus having improved signal to noise ratio.
2. Description of the Prior Art
Apparatus using nuclear magnetic resonance (NMR) are known, and especially, in the medical field, such as an NMR imaging apparatus, wherein protons in a sliced imaginary portion of an object to be imaged are excited on a principle similar to an X-ray CT to determine NMR signals corresponding to respective projections in a number of directions of the object so that the intensities of the NMR signals in the respective positions of the object may be determined by a reconstructive method.
This apparatus is operated by applying a suitable combination of magnetic fields to the object through a coil device and by detecting the NMR signals induced by the detecting coil.
The detecting coil may be a saddle shaped coil, such as shown in FIG. 1. The saddle shaped coil is known to detect only NMR signal component in a y-zxis direction perpendicular to the z direction of a principal magnetic field Ho and to have therein a relatively uniform receiving sensitivity distribution.
In the arrangement described, noises from the outside are detected together with the NMR from the object. For example, the NMR signal having a magnetic intensity of 1,000 gauss and detected from a hydrogen nuclei has a frequency of about 4.2 MHz, which is near to the frequency of noises transmitted from radio broadcasting stations, wireless communications, computers, CRT devices, etc. Moreover, automobiles, electric trains, natural phenomena, such as thunder, etc, are also source of noises. Thus, the know NMR imaging apparatus cannot generate clear clean NMR signals which are not polluted with noise, unless such apparatus are located in a shielded room. But, shielded rooms require considerable auxiliary equipment, such as air conditioning in order to be operable, and limit the wide spread use of NMR apparatus.